The present invention relates to security paper having a security zone, to a method of making such paper, and to apparatus for implementing the method.
It is known to make weakened zones in sheets of paper that are to constitute security documents, such as, for example, bank bills, means of payment, identity documents, travel tickets, or tickets for entry to cultural or sporting events. These zones are intended to receive information that must not be falsified.
The weak zone is designed in such a manner that any mechanical attack applied to its surface will give rise to irreversible damage, such as puncturing the paper or making its surface fluffy, thus making it impossible to alter any information that has already been marked in said weak zone.
One known method of making a weak zone is to apply abrasion or shaving down to half-depth in the zone, after the sheet of paper has been made, as proposed in document EP-A-0 543 528.
Making the weak zone requires an additional operation compared with the normal process of making paper, and that gives rise to extra cost that is relatively high.
Furthermore, the additional operation is difficult to industrialize, and document EP-A-0 543 528 does not provide any precise teaching on this topic.
Another known method is to form two layers of paper simultaneously by the dual-web technique, one of the layers being provided with a window where the other one does not have a window. The assembled-together layers thus provide a sheet of paper provided with a window of reduced thickness and considerable weakness.
The sheet made using the dual-web technique presents the drawback that its weak zone needs to be sufficiently weak to perform its security function while nevertheless being capable of standing up to the handling involved in industrial manufacture of the sheet. The paper manufacturer thus needs to find a good compromise between those two requirements.
Furthermore, the weak zone cannot extend over a large area since otherwise it is in danger of tearing on first use, i.e. when being used as security paper.
All of those known security papers are also vulnerable to the method of falsification that consists in shaving away a part of the thickness of the security zone and covering the remaining thickness with some other, falsified sheet, e.g. using adhesive.